On 24 August your vessel is enroute from Perth, Australia, to Bombay, India. Evening twilight will occur at 1807 zone time and your vessel's DR position will be LAT 27°17.0' S, LONG 83°17.0' E. Considering their magnitude and location, what are the three stars best suited to observe for a fix at star time?
• Star selection for evening twilight fixes (brightness, altitude, azimuth separation) • How latitude and declination affect whether a star is well-placed in the sky • Use of azimuth/altitude spread (60°–120° apart in bearing, 15°–75° in altitude) for a good 3-star fix
• From LAT 27° S, which options give you at least one or two good southern stars and not only far‑north polar stars? • Looking at each set, which three-star group is most likely to give you a wide spread in azimuth (not all in the same general direction) at evening twilight? • Which choice includes stars that are all bright "navigator" stars commonly listed near the top of star-selection tables (HO 249/229)?
• Verify which stars are 1st magnitude or very bright 2nd magnitude, since the question stresses magnitude. • Check that no star in the set has a declination so far north that, from 27° S, it would be extremely low or even below the horizon at twilight. • Confirm the chosen group would give good geometric spread: not bunched together in one sector of the sky and not all at very high or very low altitudes.
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